Two essential parameters are recognized when dealing with processing of toxic or radioactive waste materials for burial:
1. A high integrity wasteform which will not leach or decompose on exposure to burial site environment is vital. PA1 2. Volume reduction of burial wastes is important, and is especially important with radioactive and toxic wastes. The cost of burial space in licensed radioactive and toxic waste sites is very high; consequently, designers must seek cost effective techniques to reduce volume of such liquid wastes. The most effective techniques for reducing the burial volume of the waste are often effective in cost containment programs.
In dealing with aqueous waste solutions, one major goal is to find a method for removing the water. When this is accomplished, it is possible to avoid the added costs associated with burying water. An equally important consideration in dealing with radioactive and toxic wastes, namely, environmental stewardship. This dictates that the designer devise a means for solidifying the waste in a long term stable waste package.
In prior art radioactive waste processing, the preferred means for processing liquid waste solutions called for use of an evaporator which could drive off water and consequently concentrate the liquid waste to a desired salt-to-water ratio, and then use of a binder typically of portland cement to solidify the residue. The high salt concentration of the waste, however, impedes efficient heat transfer to effect evaporation and can lead to mechanical operational difficulties.
Prior techniques have presented a number of operational difficulties; namely, the mechanical complications that are synonymous with the mechanically complex operation of an evaporator unit. A secondary difficulty was experienced in dealing with the "prior art" binder systems. There was a potential for chemical incompatibility between the concentrated radioactive waste "evaporator-bottoms" and the portland cement binder which was used to form the solidified wasteform.
One popular process used previously involved mixing the concentrated aqueous waste slurry with a crosslinkable hydrophilic polymer. An example of such solidification media is an urea formaldehyde or a polyvinyl ester polymer. This process was used widely to solidify aqueous radioactive waste having high salt content. Such polymers form solid plastic composites, even in presence of water. However, the polymer/water waste composites that results from the hydrophilic polymers have been found to experience inconsistent wasteform properties due to the potentially adverse interactions between the aqueous waste concentrate and the polymer binder, especially during the polymerization process. A second problem area which is encountered with such a process is that it yields a wasteform that incorporates water in the final wasteform.
An alternative to the hydrophilic polymer involves the use of a bitumen resin into which the liquid waste is introduced, The bitumen is processed hot, thereby providing a means to volatilize water from the waste, reducing or eliminating water from the wasteform. The difficulty with the bitumen binder system rests primarily in the fact that it is impractical to operate such a system without the corresponding use of an evaporator of some design. This evaporator is used to "pre-concentrate" the aqueous waste stream. Also, the bitumens (and similar thermoplastic wasteforms) are not optimum solidification materials, because of their performance deficiencies under burial site conditions. For example, they can react with certain nitrate salts. Radiation tolerance is not good, and fire resistance is less than ideal.
Accordingly, there is a need for a process which can decrease or substantially remove the weight percent of water from aqueous waste solutions such as aqueous radioactive waste solutions. There is also a need for a process for producing a storable form of the waste, once the water has been removed. Likewise, there also is a need for storable waste forms made from aqueous waste solutions.